There are some conventional methods for producing 2,5-dichlorophenylthioglycolic acid, including the following:
(a) the method in which 1,4-dichlorobenzene is sulfochlorinated with chlorosulfonic acid and then reduced to 2,5-dichlorothiophenol with zinc powder under acidic conditions, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid (U.S. Pat. No. 3440288), PA1 (b) the method in which 2,5-dichloroaniline is diazotized and then reacted with thiourea in the presence of copper sulfate and subsequently hydrolyzed to 2,5-dichlorothiophenol, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid (Ger. Offen. DE3715508), PA1 (c) the method in which 1,2,4-trichlorobenzene and sodium hydrosulfide are reacted in the presence of copper acetate catalyst in liquid ammonia solvent under increased pressure to yield 2,5-dichlorothiophenol, followed by reaction with monochloroacetic acid to yield 2,5-dichlorophenylthioglycolic acid [Kogyo Kagaku, 70, 1384 (1967)]. PA1 (1) novel 4-carboxymethylthio-2,5-dichlorobenzenesulfonates, PA1 (2) a method for producing novel 4-carboxymethylthio-2,5dichlorobenzenesulfonates characterized by reacting 2,4,5trichlorobenzenesulfonates and thioglycolic acid in the presence of a base, PA1 (3) a method for producing 2,5-dichlorophenylthioglycolic acid characterized by desulfonating 4-carboxymethylthio-2,5dichlorobenzenesulfonates using an aqueous solution of mineral acid, and PA1 (4) a method for producing 2,5-dichlorophenylthioglycolic acid characterized by reacting 2,4,5-trichlorobenzenesulfonates and thioglycolic acid in the presence of a base and subsequently reacting the reaction mixture with mineral acid or an aqueous solution of mineral acid. PA1 (1) the use of 2,4,5-trichlorobenzenesulfonates as a starting material offers improvement in the reactivity of the 4-position chlorine atom due to the strong electron attracting property of the sulfone group and thus makes reaction with thioglycolic acid in the presence of base easy and permits direct introduction of thioglycol group (Process A), and ##STR1## (wherein M represents hydrogen, sodium or potassium) (2) the novel 4-carboxymethylthio-2,5-dichlorobenzenesulfonates thus obtained, which are 2,5-dichlorophenylthioglycolic acid derivatives, can easily be desulfonated to the desired 2,5-dichlorophenylthioglycolic acid by reacting with an aqueous solution of mineral acid due to the action of thioglycol group, which is an electron donor (Process B). The reaction proceeds with high yield in both processes.
However, these known methods respectively have the following drawbacks.
In the method of (a) above, a large amount of waste effluent which contains harmful heavy metals poses a major problem of environmental pollution. In the method of (b) above, the copper compound used is difficult to dispose of as in (a) and the production efficiency is low; it is not an economic method of production. In the method of (c) above, liquid ammonia is difficult to handle and the copper compound used is difficult to dispose of, and additionally special equipment is required because the reaction is carried out under high pressure. Also, there is a problem of very low yield.
As stated above, all these known methods have various drawbacks and are not industrially advantageous,
For this reason, attempts have been made to develop an industrially advantageous method for producing 2,5-dichlorophenylthioglycolic acid, but there is no satisfactory method.